Transmission of reference signals from a terminal device

ABSTRACT

There is provided mechanisms for transmission of reference signals. A method is performed by a terminal device. The terminal device comprises at least two physical antenna ports. The method comprises obtaining channel information. The method comprises determining, for at least one given precoder in a codebook, an uplink reference signal to physical antenna port mapping based on the channel information. The method comprises transmitting uplink reference signals in the physical antenna ports according to the determined mapping.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.16/472,048, filed Jun. 20, 2019 and now issued as U.S. Pat. No.11,411,695, which is a 35 U.S.C. § 371 National Phase Entry Applicationfrom PCT/EP2019/060942, filed Apr. 29, 2019, designating the UnitedStates. The disclosures of each of the referenced applications areincorporated herein in their entirety by reference.

TECHNICAL FIELD

Embodiments presented herein relate to a method, a terminal device, acomputer program, and a computer program product for transmission ofreference signals.

BACKGROUND

In communications networks, there may be a challenge to obtain goodperformance and capacity for a given communications protocol, itsparameters and the physical environment in which the communicationsnetwork is deployed.

For example, equipping a terminal device with two or more, or evenmultiple, transmit antennas (where each such antenna is connected to itsown physical antenna port at the terminal device) might enable theuplink performance to be increased through higher spectral efficiencyand/or improved link budget than allowed for only one single antenna.

Two uplink transmission schemes will be considered next; codebook based(CB) transmission and non-codebook based (NCB) transmission. CBtransmission is a feedback based transmission scheme that can be usedfor frequency division duplex (FDD) and for terminal devices withouttransmitter-receiver reciprocity. NCB transmission is based onreciprocity and can be used in time division duplex (TDD) for UEs withtransmitter-receiver reciprocity.

In CB transmission the terminal device first transmits one or two uplinkreference signals, such as sounding reference signals (SRS, or SRSports). The radio access network node serving the terminal deviceestimates the uplink radio propagation channel based on the receiveduplink reference signals, where each uplink reference signal istransmitted from a separate physical antenna port, and determines asuitable transmission rank and precoder for the coming uplink datatransmission. In general terms, the number of rows of the precoderequals the number of physical antenna ports, and the number of columnsof the precoders equals the number of layers. The precoder could beselected from a predetermined set of fixed precoders defined in the 3GPPspecifications, a so-called codebook, see 3GPP TS 38.211 “NR; Physicalchannels and modulation”, Version 15.5.0. The radio access network nodethen signals the transmission rank via a transmit rank indicator (TRI)and an index to the determined precoder in the codebook, a so-calledtransmit precoder matrix indicator (TPMI). The terminal device shallthen use the precoder corresponding to the signaled TRI and TPMI in itsupcoming uplink data transmission.

Depending on terminal device implementation, it may be possible tomaintain the relative phase of the transmit chains of an antenna array(assuming for example one radio chain per antenna element). In thiscase, the terminal device is enabled to transmit the same modulationsymbol over multiple transmit chains with individual gain and/or phaseper transmit chain and in this way forming a beam over the correspondingantenna array. This transmission of a common modulation symbol or signalon multiple antenna elements with controlled phase is referred to ascoherent transmission. The support for coherent uplink MIMO transmissionin Release 10 of the Long Term Evolution (LTE) suite oftelecommunication standards is indicated via a feature group indicationfor relative transmit phase continuity for uplink spatial multiplexing,wherein a terminal device indicates if it can adequately maintain therelative phase of transmit chains over time in order to support coherenttransmission.

In other terminal device implementations, the relative phase of thetransmit chains may not be well controlled, and coherent transmissionmay not be used. In such implementations, it may still be possible forthe terminal device to transmit on one of the transmit chains at a time,or to transmit different modulation symbols on the transmit chains. Inthe latter case, the modulation symbols on each transmit chain may forma spatially multiplexed, or MIMO, layer. This class of transmission isreferred to as non-coherent transmission.

In still other terminal device implementations, the relative phase of asubset of the transmit chains is well controlled, but not over alltransmit chains. One possible example is described above with respect tomulti-panel operation, where phase is well controlled among transmitchains within a panel, but phase between panels is not well controlled.This class of transmission is referred to as partially-coherent.

All three of these variants of relative phase control have been agreedto be supported for transmission over the fifth generation (5G) NewRadio (NR) air interface, and so terminal device capabilities have beendefined for full coherence, partial coherence, and non-coherenttransmission. Depending on coherence capability of the terminal device,it is possible to configure the terminal device with three differentcombinations of codebook subsets. FIG. 1 illustrates three differentcodebook subsets 10, 20, 30 for rank 1 precoders. Codebook subset 10 isreferred to as non-coherent and only consists of antenna selectionprecoders. Codebook subset 20 is referred to as partial-coherent andonly consists of antenna pair selection precoders. Codebook subset 30 isreferred to as fully-coherent and only consists of full linear combiningprecoders. Depending on the terminal device coherence capability, theradio access network node can configure the terminal device with threedifferent combinations of the codebook subsets. For non-coherentterminal devices, the radio access network node is expected to configurethe terminal device with only the non-coherent codebook subset 10. Forpartially-coherent terminal devices, the radio access network node isexpected to configure the terminal device with both the non-coherent andthe partial-coherent codebook subsets 10, 20, and for fully-coherentterminal devices, the radio access network node is expected to configurethe terminal device with all three codebook subsets 10, 20, 30.

How the uplink reference signals should be transmitted, for examplewhich uplink reference signal resources to use, frequency allocation,time domain behavior (periodic, semi-persistent or aperiodic), etc.,needs to be signaled to the terminal device from the radio accessnetwork node. One way to implement this is to define a number of uplinkreference signal resource sets using higher layer signaling (such asradio resource control (RRC) signaling), where each uplink referencesignal resource set contains a list of uplink reference signalresources. In some communication systems only one uplink referencesignal resource set is defined, which might contain up to two uplinkreference signal resources, where each uplink reference signal resourcemight consist of up to four ports (such as SRS ports). The transmissionof the uplink reference signals is then triggered by letting the radioaccess network node signal a pointer to this uplink reference signalsresource set, which implicitly indicates to the terminal device totransmit the uplink reference signal resources listed in that uplinkreference signal resource set.

The codebooks for codebook based uplink transmission for the 5G NR airinterface are specified in the aforementioned document 3GPP TS 38.211. Arow in a precoding matrix corresponds to a particular SRS port and acolumn corresponds to a layer in the PUSCH transmission.

When applying the codebook to a terminal device with more than onephysical antenna port, it must be decided in which physical antenna porta particular uplink reference signal (such as a SRS port) should betransmitted. For example, SRS port 1 could be transmitted in physicalantenna port 1, SRS port 2 in physical antenna port 2, and so on. Thesame mapping is then used in the next upcoming uplink data transmission.

However, the 4-port codebook specified in the aforementioned document3GPP TS 38.211 is incomplete with regards to how the four physicalantenna ports at the terminal device are to be used for uplink datatransmission. In particular, the 4-port codebook does not support allantenna port combinations, which, for example, can result in inefficientuse of the antenna array and limited transmission power budget.

Hence, there is still a need for an improved codebook based uplinktransmission.

SUMMARY

An object of embodiments herein is to provide efficient transmission ofreference signals enabling improved codebook based uplink transmission.

According to a first aspect there is presented a method for transmissionof reference signals. The method is performed by a terminal device. Theterminal device comprises at least two physical antenna ports. Themethod comprises obtaining channel information. The method comprisesdetermining, for at least one given precoder in a codebook, an uplinkreference signal to physical antenna port mapping based on the channelinformation. The method comprises transmitting uplink reference signalsin the physical antenna ports according to the determined mapping.

According to a second aspect there is presented a terminal device fortransmission of reference signals. The terminal device comprisesprocessing circuitry. The processing circuitry is configured to causethe terminal device to obtain channel information. The processingcircuitry is configured to cause the terminal device to determine, forat least one given precoder in a codebook, an uplink reference signal tophysical antenna port mapping based on the channel information. Theprocessing circuitry is configured to cause the terminal device totransmit uplink reference signals in the physical antenna portsaccording to the determined mapping.

According to a third aspect there is presented a terminal device fortransmission of reference signals. The terminal device comprises anobtain module configured to obtain channel information. The terminaldevice comprises a determine module configured to determine, for atleast one given precoder in a codebook, an uplink reference signal tophysical antenna port mapping based on the channel information. Theterminal device comprises a transmit module configured to transmituplink reference signals in the physical antenna ports according to thedetermined mapping.

According to a fourth aspect there is presented a computer program fortransmission of reference signals, the computer program comprisingcomputer program code which, when run on a terminal device, causes theterminal device to perform a method according to the first aspect.

According to a fifth aspect there is presented a computer programproduct comprising a computer program according to the fourth aspect anda computer readable storage medium on which the computer program isstored. The computer readable storage medium could be a non-transitorycomputer readable storage medium.

Advantageously this provides efficient transmission of referencesignals.

Advantageously this enables improved codebook based uplink transmission.

Advantageously this enables better usage of the physical antenna portsduring codebook based uplink transmission, In turn, this can increasethe uplink performance.

Other objectives, features and advantages of the enclosed embodimentswill be apparent from the following detailed disclosure, from theattached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, module, step, etc.” are to be interpretedopenly as referring to at least one instance of the element, apparatus,component, means, module, step, etc., unless explicitly statedotherwise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 schematically illustrates precoders;

FIGS. 2 and 5 are schematic diagrams illustrating communication systemsaccording to embodiments;

FIG. 3 schematically illustrates a terminal device according to anembodiment;

FIG. 4 is a flowchart of methods according to embodiments;

FIG. 6 is a schematic diagram showing functional units of a terminaldevice according to an embodiment;

FIG. 7 is a schematic diagram showing functional modules of a terminaldevice according to an embodiment; and

FIG. 8 shows one example of a computer program product comprisingcomputer readable storage medium according to an embodiment.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter withreference to the accompanying drawings, in which certain embodiments ofthe inventive concept are shown. This inventive concept may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the inventive concept tothose skilled in the art. Like numbers refer to like elements throughoutthe description. Any step or feature illustrated by dashed lines shouldbe regarded as optional.

FIG. 2 is a schematic diagram illustrating a communication system bowwhere embodiments presented herein can be applied. The communicationssystem bow comprises a radio access network node 140 a configured toprovide network access over one or more radio propagation channels to aterminal device 200 in a radio access network no. Non-limited examplesof terminal devices 200 are portable wireless devices, mobile stations,mobile phones, handsets, wireless local loop phones, user equipment(UE), smartphones, laptop computers, tablet computers, network equippedsensors, network equipped vehicles, and Internet of Things (IoT)devices. In some embodiments the radio access network node 140 a is partof, integrated with, or collocated with a radio base station, basetransceiver station, node B, evolved node B, gNB, access point, or thelike. The radio access network 110 is operatively connected to a corenetwork 120. The core network 120 is in turn operatively connected to apacket data network 130, such as the Internet. The terminal device 200is thereby, via the radio access network node moa, enabled to accessservices of, and exchange data with, the service network 130.

FIG. 3 schematically illustrates a terminal device 200 equipped withfour physical antennas 260. As the skilled person understands, these arejust examples and the terminal device 200 might be equipped with more(or less) physical antennas 260. Each physical antenna 260 has its ownpower amplifier (PA) 270. That is, each physical antenna port 250 is fedby its own PA 270. Each physical antenna 260 is connected to basebandcircuitry 280 via its own physical antenna port 250. Thereby, when theterminal device 200 transmits reference signals, each of the referencesignals comes from a respective one of the physical antenna ports 250.In some aspects the terminal device 200 has four or eight physicalantenna ports 250 in total (depending on the number of physical antennas260). Although each physical antenna 260 in FIG. 3 is illustrated ascomprising only one single antenna element, as the skilled personunderstands, each physical antenna 260 might be implemented as an arrayof antenna elements. That is, each physical antenna port 250 could beoperatively connected to only a single antenna element or an array of atleast two antenna elements. In the illustrative example of FIG. 3 , theantenna element or array of at least two antenna elements of at leasttwo of the physical antenna ports 250 are arranged at the terminaldevice 200 to point in at least two mutually different pointingdirections 290 a, 290 b, 290 c. Two of the physical antennas 260 pointin direction 290 a, and a respective one of the physical antennas 260points in directions 290 b and 290 c. Depending on the carrierfrequencies the antenna element radiation patterns (hereinafterradiation pattern for short) given rise to by transmission (andreception) in the physical antenna ports 250 might be more or lessdirectional. For lower frequencies the radiation patterns are typicallyfairly omni-directional, but when the carrier frequency increases theradiation pattern typically becomes more and more directional.

As noted above, there is a need for an improved codebook based uplinktransmission.

In more detail, the usefulness of transmitting from a particularphysical antenna port (or, combination of physical antenna ports) at anygiven time will depend on the channel conditions at that time and theantenna configuration at the terminal device 200. In general terms, theantenna configuration is defined by how the antenna elements arepositioned on the terminal device 200 and their radiation patterns. Thechannel conditions will change over time and, therefore, the usefulnessof each physical antenna port (or, combination of physical antennaports) will also change over time.

Furthermore, it might be challenging to determine a priori the optimaluplink reference signal to physical antenna port mapping based on theantenna configuration. For example, it might be challenging to determinean antenna configuration for a handheld terminal device with preciselycontrolled gain and polarization properties and, therefore, theseproperties will vary from terminal device to terminal device.

For rank 1, the partially coherent TPMIs 4-11 of the precoding matricesspecified in the aforementioned document 3GPP TS 38.211 implies usage ofeither physical antenna ports 1 and 3 or physical antenna ports 2 and 4,if the mapping between uplink reference signals and physical antennaports is defined by the identity matrix is used. However, it might bebetter to instead use physical ports 1 and 2 or physical antenna ports 3and 4, depending on the channel conditions. The rank 2 TPMIs 6-13 defineprecoding matrices of the partially coherent codebook subset. For theseTPMIs, the first layer is transmitted over physical antenna ports 1 and3 and the second layer is transmitted over physical antenna ports 2 and4. For rank 3 there is a TPMI (TPMI 0) that does not use physicalantenna port 4. If one of the other physical antenna ports (i.e., any ofphysical antenna ports 1-3) is blocked, it should be better to remap theblocked physical antenna port to SRS port 4 since SRS port 4 is not usedanyway.

In summary, depending on the antenna configuration and the channelconditions, it could happen that none of the TPMIs in the 4-portcodebook is optimal. There is thus potential for increased performanceif a precoder not present in the 4-port codebook can be enabled, withoutintroducing additional control overhead.

The embodiments disclosed herein therefore relate to mechanisms fortransmission of reference signals. In order to obtain such mechanismsthere is provided a terminal device 200, a method performed by theterminal device 200, a computer program product comprising code, forexample in the form of a computer program, that when run on a terminaldevice 200, causes the terminal device 200 to perform the method.

FIG. 4 is a flowchart illustrating embodiments of methods fortransmission of reference signals. The methods are performed by theterminal device 200. The terminal device 200 comprises at least twophysical antenna ports 250. The methods are advantageously provided ascomputer programs 820.

According to the herein disclosed embodiments precoders missing from theaforementioned document 3GPP TS 38.211 can be realized by the terminaldevice 200 being configured to dynamically change/control/configure itsuplink reference signal (which might define an SRS port) to physicalantenna port mapping.

The terminal device 200 is configured to determine an uplink referencesignal to physical antenna port mapping based on channel information.Thus, the terminal device 200 is configured to perform S102 and S104:

S102: The terminal device 200 obtains channel information.

Different examples of channel information will be disclosed below.

S104: The terminal device 200 determines, for at least one givenprecoder in a codebook, the uplink reference signal to physical antennaport mapping based on the channel information.

In this respect, the terminal device 200 might thus determine themapping based on two or more precoders. Different ways in which themapping might be determined based on the channel information will bedisclosed below. The determined mapping is then used for transmission ofuplink reference signals. Thus, the terminal device 200 is configured toperform S106:

S106: The terminal device 200 transmits uplink reference signals in thephysical antenna ports 250 according to the determined mapping.

Advantageously, this enables an adaptive mapping of SRS ports (as givenby the uplink reference signals) to physical antenna ports 250 in theterminal device for codebook based uplink precoding based onmeasurements of channel conditions, as defined by the obtained channelinformation. For example, if the terminal device 200 determines that aphysical antenna port is weak (e.g., by being blocked), then theterminal device 200 might dynamically configure its mapping to avoidusing this physical antenna port during upcoming uplink datatransmission, i.e., at least during the next-most occurring uplink datatransmission.

Embodiments relating to further details of transmission of referencesignals as performed by the terminal device 200 will now be disclosed.

There may be different ways to determine the uplink reference signal tophysical antenna port mapping (hereinafter denoted mapping). Differentembodiments relating thereto will now be described in turn.

In some aspects the terminal device 200 measures on downlink referencesignals. That is, according to an embodiment, the channel informationobtained from measurements on downlink reference signals. Non-limitingexamples of downlink reference signals are channel state informationreference signals (CSI-RSs), synchronization signal blocks (SSBs), etc.

In some aspects the terminal device 200 measures the channel correlationbetween all physical antenna port pairs in order to determine whichantenna ports that should be used for the same layer. In particular,according to an embodiment, the channel information pertains to channelcorrelation or average received signal power. The number of layers isdeduced from the channel correlation and the received power. Further,according to an embodiment, the at least one precoder definestransmission on at least one spatial layer, and the mapping isdetermined to select which at least one spatial layer to be transmittedon which physical antenna port 250.

Since low correlation is advantageous for spatial multiplexing and highcorrelation is advantageous for beamforming, physical antenna ports 250with low correlation might be mapped to reference signal ports used fortransmitting different layers and physical antenna ports 250 with highcorrelation might be mapped to reference signal ports for the samelayer. In particular, according to an embodiment, the terminal device200 is configured to perform (optional) step S104 a as part ofdetermining the mapping in S104:

S104 a: The terminal device 200 determines channel correlation betweenall pairs of the physical antenna ports 250. Those pairs of the physicalantenna ports 250 having highest channel correlation are then mapped tothe same spatial layer in the precoder. As an example, for TPMI 6-13 inTable 6.3.1.5-5 of aforementioned document 3GPP TS 38.211, SRS ports 1and 3 should be mapped to one antenna port pair with high correlationand SRS ports 2 and 4 should be mapped to another antenna port pair withhigh correlation. Alternatively, the mapping might be performed suchthat the mutual correlation between the physical antenna portscorresponding to SRS ports 1 and 3 and SRS ports 2 and 4 becomes as lowas possible.

In some aspects there is at least one layer where the coefficients arezero. In particular, according to an embodiment, at least one row butless than all rows in each of the at least one precoder consists of allzero-valued coefficients. At least one, but not all, row(s) of theprecoder contains all-zeros (i.e., where all coefficients have the valuezero). This row with all-zeros should not be mapped to the physicalantenna port with the highest received power.

In some aspects the mapping is determined based on pairing physicalantenna ports 250 which have highest average received signal power. Inparticular, according to an embodiment, the terminal device 200 isconfigured to perform (optional) step S104 b as part of determining themapping in S104:

S104 b: The terminal device 200 determines average received signal powerfor all pairs of the physical antenna ports 250. Those pairs of thephysical antenna ports 250 having highest average received signal powerare then mapped to spatial layers having non-all zero-valuedcoefficients in the at least one precoder.

According to an example, assuming that the terminal device 200 isequipped with four physical antenna ports, if two physical antenna portsreceive significantly higher power than the other two physical antennaports the terminal device 200 maps SRS port 1 and 3 or SRS ports 2 and 4to the two physical antenna ports in which significantly higher power isreceived. This is because, according to aforementioned document 3GPP TS38.211, for the rank 1 precoders in the codebook that use two SRS ports,one layer is mapped either to SRS ports 1 and 3 or to SRS ports 2 and 4.For rank 2, the mapping does not matter if one SRS port per layer isused since there are no missing precoders for that case.

In some aspects the mapping is determined based on an estimate ofexpected transmission rank in upcoming uplink data transmission. Inparticular, according to an embodiment, the terminal device 200 isconfigured to perform (optional) step S104 c as part of determining themapping in S104:

S104 c: The terminal device 200 estimates expected transmission rank forupcoming uplink data transmission from the channel correlation oraverage received signal power. The mapping is then based on the expectedtransmission rank.

Further, the expected transmission rank could also be estimated based onstatistics of previous uplink transmissions. For example, iftransmission of a certain rank has been used for the latest most one ormore uplink transmissions, then it could be expected that this wouldalso be the expected transmission rank for the upcoming uplink datatransmission.

According to an example, the terminal device 200 might thus estimate theexpected transmission rank in coming PUSCH transmission. This can beachieved by the terminal device 200 analyzing received signal strengthand channel correlation. If the received signal strength is high andchannel correlation is low, a high transmission rank (such as rank 3 or4) is expected, and otherwise a low transmission rank (such as rank 1 or2) is expected. This analysis does not require channel reciprocity. Ifreciprocity holds, the terminal device 200 might perform a more detailedanalysis and calculate expected throughput for each TPMI in the codebookbased on an estimated channel matrix and then estimate the transmissionrank as the one giving the highest throughput. The terminal device 200might then use the estimated rank to optimize the mapping for this rank.For example, if rank 2 is estimated as the most likely transmission rankfor the upcoming uplink data transmissions, the mapping might beoptimized for the rank 2 precoders according to the embodiment involvingabove defined S104 a.

In some aspects the mapping is determined to avoid using a weak physicalantenna port during upcoming uplink data transmission. In particular,according to an embodiment, those pairs of the physical antenna ports250 having lowest average received signal power are mapped to spatiallayers having all zero-valued coefficients in the at least one precoder.That is, according to an example, if one physical antenna port 250 hasbeen detected as being blocked, e.g., by measuring low received power,and rank 3 PUSCH transmission is anticipated, SRS port 4 is mapped tothe blocked antenna port. This is because the precoding matrix of TPMI 0for rank 3 does not use SRS port 4, see Table 6.3.1.5.4-6 in theaforementioned document 3GPP TS 38.211.

In some aspects, the mapping is frequency selective such that differentmappings are applied in different frequency subbands. In particular,according to an embodiment, the terminal device 200 is configured tooperate in at least two frequency bands, and the mapping is frequencyselective such that different mappings are applied in the at least twofrequency bands.

In some aspects, uplink data transmission from the terminal device 200following the transmission of the reference signals will utilize thedetermined mapping. Particularly, according to an embodiment, theterminal device 200 is configured to perform (optional) step S108:

S108: The terminal device 200 transmits uplink data in the physicalantenna ports (250) using one of the at least one given codebook basedprecoder as applied according to the determined mapping.

There could be different examples of uplink data. In some examples theuplink data is transmitted on a physical uplink shared channel (PUSCH).A terminal device 200 might thus transmit PUSCH using a codebook basedprecoder as applied according to the determined mapping.

There could be different examples of uplink reference signals. Accordingto an example, the uplink reference signals are sounding referencesignals (SRS). In some aspects the uplink reference signals aretransmitted over the 5G NR air interface.

The herein disclosed embodiments could be applied to both time divisionduplex (TDD) systems with channel reciprocity and frequency divisionduplex (FDD) systems without channel reciprocity. For FDD systems, theterminal device 200 might be configured to determine that a physicalantenna port is blocked using, for example, wideband statistics fromdownlink reference signals (so as to avoid fast-fading issues withdifferent carriers). Blockage of a physical antenna port might also bedetected by other means, e.g., impedance/reflection measurements,proximity sensors, etc.

The herein disclosed embodiments could be applied when it is challengingto determine the best mapping a priori based on properties of theantennas (such as the antenna configuration), which is often the casesince it might be challenging to design antennas for a handheld terminaldevice with precisely controlled gain and polarization properties. Sincethe herein disclosed embodiments are based on channel conditions, themapping does not rely on any assumptions made on the antenna properties.

FIG. 5 at (a), (b), (c), and (d) gives four examples of communicationsystems 100 b, 100 c, 100 d, woe where a terminal device 200 performsuplink reference signal to physical antenna port mapping in accordancewith the herein disclosed embodiments. The examples are simplificationsin order to emphasize the herein proposed inventive concept moreclearly. In reality, the antenna radiation patterns and polarizationstates of the terminal device 200 are more irregular and random innature and the radio propagation channel might be characterized byclusters of different radio signal strengths. However, since theproposed inventive concept is based on obtaining channel information andnot on a priori assumptions thereof, the examples of FIG. 5 are validalso under more realistic assumptions. The terminal device comprisesfour physical antenna ports, identified at 1, 2, 3, and 4. One schematicradiation pattern 150 is illustrated at each physical antenna port. Eachuplink reference signal is represented by an SRS port.

In all examples it is assumed that the precoding matrix given by any ofTPMI 4, 5, 6, or 7 of Table 6.30.1.5-3 in the aforementioned document3GPP TS 38.211 is to be used by the terminal device 200 during uplinktransmission.

In FIG. 5(a), the terminal device 200 is located between two radioaccess network nodes 140 a, 140 b where the radiation patterns forphysical antenna ports 1 and 3 have strong gain towards radio accessnetwork node 140 a and the radiation patterns for physical antenna ports2 and 4 have high gain towards radio access network node mob. In thiscase, SRS ports 1 and 3 should be mapped to physical antenna ports 1 and3 and SRS ports 2 and 4 should be mapped to physical antenna ports 2 and4, according to the previous discussion.

In FIG. 5(b), the terminal device 200 has been rotated 90 degrees inrelation to FIG. 5(b) so that physical antenna ports 1 and 2 now havehigh gain towards radio access network node 140 a whereas physicalantenna ports 3 and 4 have high gain towards radio access network nodemob. In this case, SRS ports 1 and 3 should be mapped to physicalantenna ports 1 and 2 and SRS ports 2 and 4 should be mapped to antennaports 3 and 4.

In FIG. 5(c), there is only one radio access network node 140 a towardswhich antennas port 1 and 2 have high gain. In this case, SRS ports 1and 3 should be mapped to physical antenna ports 1 and 2 and SRS ports 2and 4 should be mapped to physical antenna ports 3 and 4.

In FIG. 5(d), physical antenna ports 3 and 4 are blocked so that thesephysical antenna ports receive with very low power. In this case, SRSports 1 and 3 should be mapped to physical antenna ports 1 and 2 and SRSport 2 and 4 should be mapped to physical antenna ports 3 and 4.

As the skilled person, although some of the above examples have beengiven for a terminal device 200 with four physical antenna ports andreference therefore has been made to the 4-port codebooks, the hereindisclosed principles apply also to other number of physical antennaports and also other codebooks and precoding matrices than thosespecified in the aforementioned document 3GPP TS 38.211.

FIG. 6 schematically illustrates, in terms of a number of functionalunits, the components of a terminal device 200 according to anembodiment. Processing circuitry 210 is provided using any combinationof one or more of a suitable central processing unit (CPU),multiprocessor, microcontroller, digital signal processor (DSP), etc.,capable of executing software instructions stored in a computer programproduct 810 (as in FIG. 8 ), e.g. in the form of a storage medium 230.The processing circuitry 210 may further be provided as at least oneapplication specific integrated circuit (ASIC), or field programmablegate array (FPGA).

Particularly, the processing circuitry 210 is configured to cause theterminal device 200 to perform a set of operations, or steps, asdisclosed above. For example, the storage medium 230 may store the setof operations, and the processing circuitry 210 may be configured toretrieve the set of operations from the storage medium 230 to cause theterminal device 200 to perform the set of operations. The set ofoperations may be provided as a set of executable instructions.

Thus the processing circuitry 210 is thereby arranged to execute methodsas herein disclosed. The storage medium 230 may also comprise persistentstorage, which, for example, can be any single one or combination ofmagnetic memory, optical memory, solid state memory or even remotelymounted memory. The terminal device 200 may further comprise acommunications interface 220 at least configured for communications withother entities, nodes, functions, and devices of the communicationsystem 100. As such the communications interface 220 may comprise one ormore transmitters and receivers, comprising analogue and digitalcomponents. The processing circuitry 210 controls the general operationof the terminal device 200 e.g. by sending data and control signals tothe communications interface 220 and the storage medium 230, byreceiving data and reports from the communications interface 220, and byretrieving data and instructions from the storage medium 230. Othercomponents, as well as the related functionality, of the terminal device200 are omitted in order not to obscure the concepts presented herein.

FIG. 7 schematically illustrates, in terms of a number of functionalmodules, the components of a terminal device 200 according to anembodiment. The terminal device 200 of FIG. 7 comprises a number offunctional modules; an obtain module 210 a configured to perform stepS102, a determine module 210 b configured to perform step S104, and atransmit module 210 f configured to perform step S106. The terminaldevice 200 of FIG. 7 may further comprise a number of optionalfunctional modules, such as any of a determine module 210C configured toperform step S104 a, a determine module 210 d configured to perform stepS104 b, an estimate module 210 e configured to perform step S104 c, anda transmit module 210 g configured to perform step S108.

In general terms, each functional module 210 a-210 g may in oneembodiment be implemented only in hardware and in another embodimentwith the help of software, i.e., the latter embodiment having computerprogram instructions stored on the storage medium 230 which when run onthe processing circuitry makes the terminal device 200 perform thecorresponding steps mentioned above in conjunction with FIG. 7 . Itshould also be mentioned that even though the modules correspond toparts of a computer program, they do not need to be separate modulestherein, but the way in which they are implemented in software isdependent on the programming language used. Preferably, one or more orall functional modules 210 a-210 g may be implemented by the processingcircuitry 210, possibly in cooperation with the communications interface220 and/or the storage medium 230. The processing circuitry 210 may thusbe configured to from the storage medium 230 fetch instructions asprovided by a functional module 210 a-210 g and to execute theseinstructions, thereby performing any steps as disclosed herein.

Examples of terminal devices 200 have been given above.

FIG. 8 shows one example of a computer program product 810 comprisingcomputer readable storage medium 830. On this computer readable storagemedium 830, a computer program 820 can be stored, which computer program820 can cause the processing circuitry 210 and thereto operativelycoupled entities and devices, such as the communications interface 220and the storage medium 230, to execute methods according to embodimentsdescribed herein. The computer program 820 and/or computer programproduct 810 may thus provide means for performing any steps as hereindisclosed.

In the example of FIG. 8 , the computer program product 810 isillustrated as an optical disc, such as a CD (compact disc) or a DVD(digital versatile disc) or a Blu-Ray disc. The computer program product810 could also be embodied as a memory, such as a random access memory(RAM), a read-only memory (ROM), an erasable programmable read-onlymemory (EPROM), or an electrically erasable programmable read-onlymemory (EEPROM) and more particularly as a non-volatile storage mediumof a device in an external memory such as a USB (Universal Serial Bus)memory or a Flash memory, such as a compact Flash memory. Thus, whilethe computer program 820 is here schematically shown as a track on thedepicted optical disk, the computer program 820 can be stored in any waywhich is suitable for the computer program product 810.

The inventive concept has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the inventive concept, as definedby the appended patent claims.

The invention claimed is:
 1. A method for transmission of referencesignals, the method being performed by a terminal device, the terminaldevice comprising at least two physical antenna ports, the methodcomprising: determining a mapping between an uplink reference signal anda physical antenna port based on channel correlation or average receivedsignal power; and based on the determined mapping, transmitting anuplink reference signal using one or more of said at least two physicalantenna port.
 2. the method according to claim 1, wherein determiningthe mapping comprises estimating expected transmission rank for upcominguplink data transmission from the channel correlation or averagereceived signal power, and the mapping is based on the expectedtransmission rank.
 3. The method according to claim 1, wherein themapping is for at least one precoder in a codebook, the at least oneprecoder defines transmission of at least one spatial layer, and themapping is determined to select one or more of said at least twophysical antenna ports to use for transmitting said at least one spatiallayer.
 4. The method according to claim 3, wherein determining themapping comprises determining channel correlation of each pair ofphysical antenna ports included in said at least two physical antennaports, and a pair of physical antenna ports that is included in said atleast two physical antenna ports and that has highest channelcorrelation is mapped to the same spatial layer in the precoder.
 5. Themethod according to claim 4, wherein determining the mapping comprisesdetermining average received signal power of each pair of physicalantenna ports included in said at least two physical antenna ports, anda pair of physical antenna ports that is included in said at least twophysical antenna ports and that has the highest average received signalpower is mapped to at least one spatial layer having non-all zero-valuedcoefficients in the at least one precoder.
 6. The method according toclaim 5, wherein a pair of physical antenna ports that is included insaid at least two physical antenna ports and that has the lowest averagereceived signal power is mapped to at least one spatial layer having allzero-valued coefficients in the at least one precoder.
 7. The methodaccording to claim 3, wherein at least one row but less than all rows inthe at least one precoder consists of all zero-valued coefficients. 8.The method according to claim 3, further comprising: transmitting uplinkdata in the at least two physical antenna ports using one of the atleast one given codebook based precoder as applied according to thedetermined mapping.
 9. The method according to claim 8, wherein theuplink data is transmitted on a physical uplink shared channel, PUSCH.10. The method according to claim 1, wherein the terminal device isconfigured to operate in at least two frequency bands, and the mappingis frequency selective such that different mappings are applied in theat least two frequency bands.
 11. The method according to claim 1,wherein each of the at least two physical antenna ports is fed by itsown power amplifier.
 12. The method according to claim 1, wherein eachof the at least two physical antenna ports is operatively connected toonly a single antenna element or an array of at least two antennaelements.
 13. The method according to claim 12, wherein the antennaelement or the array of at least two antenna elements of at least two ofthe physical antenna ports are arranged at the terminal device to pointin at least two mutually different pointing directions.
 14. The methodaccording to claim 1, wherein each uplink reference signal defines asounding reference signal (SRS) port.
 15. The method according to claim1, wherein the uplink reference signal is transmitted over 5G New Radio(NR) air interface.
 16. A terminal device for transmission of referencesignals, the terminal device comprising at least two physical antennaports and processing circuitry, the processing circuitry beingconfigured to cause the terminal device to: determine a mapping betweenan uplink reference signal and a physical antenna port based on channelcorrelation or average received signal power; and based on thedetermined mapping, transmit an uplink reference signal using one ormore of said at least two physical antenna ports.
 17. A computer programproduct comprising a non-transitory computer readable medium storing acomputer program comprising instructions which, when run on processingcircuitry of a terminal device comprising at least two physical antennaports causes the terminal device to: determine a mapping between anuplink reference signal and a physical antenna port based on channelcorrelation or average received signal power; and based on thedetermined mapping, transmit an uplink reference signal using one ormore of said at least two physical antenna ports.